Abstract
Abstract The Freiberg district hosts one of the largest series of epithermal polymetallic vein deposits in Europe. The availability of a systematic collection of historical samples provides an excellent opportunity to study the anatomy of these epithermal systems. Detailed petrographic investigations, geochemical analyses, and fluid inclusion studies were conducted on several vertical profiles within the Freiberg district to decipher mineralogical and geochemical zoning patterns. Six distinctive mineral associations have been recognized within the Freiberg epithermal veins; sphalerite-pyrite-quartz and galena-quartz±carbonate associations are most abundant in the central sector, as well as in the deepest sections of veins on the periphery of the district. A high-grade sphalerite-Ag-sulfides-carbonate association occurs laterally between the central and peripheral sectors and at intermediate depth in veins on the periphery. Shallow and peripheral zones are dominated by an exceptionally Ag-rich Ag-sulfides-quartz association, whereas the shallowest veins locally comprise Ag-poor stibnite-quartz and quartz-carbonate associations. Fluid inclusion assemblages returned low salinities (<6.0 wt % NaCl equiv), and homogenization temperatures successively decrease from ~320°C associated with the proximal and deep sphalerite-pyrite-quartz association, to ~170°C related to the distal and shallow Ag-sulfides-quartz association. The architecture of the Freiberg district is related to the temporal and spatial evolution of magmatic-hydrothermal fluid systems, including boiling and concomitant cooling, as well as CO2 loss. Constraints on the paleodepth indicate that the veins formed between 200 and 1,800 m below the paleowater table. High-grade Ag ore occurs over a vertical interval of at least 500 m and is bracketed by shallower stibnite-quartz and barren quartz, and deeper base metal-sulfide-quartz zones.
Highlights
Intermediate sulfidation Ag-Pb-Zn epithermal systems are a major source of Ag and contain economic amounts of Au, Zn, Pb, and Cu (Simmons et al, 2005)
Many of the wellknown examples of this particular ore deposit type are located in the Sierra Madre Occidental of Mexico, e.g., Fresnillo, Tayoltita, and Pachuca-Real del Monte (Simmons, 1991; Albinson et al, 2001; Camprubí and Albinson, 2007), with similar epithermal deposits occurring in Peru (Petersen et al, 1977; Candiotti de los Rios et al, 1990; Baumgartner et al, 2008; Rottier et al, 2018) and Bolivia (Phillipson and Romberger, 2004; Arce Burgoa, 2009), as well as in Spain (Concha et al, 1992), Australia (Oliver et al, 2019), and elsewhere (Sillitoe and Hedenquist, 2003). Many of these deposits have a distinct vertical and lateral zoning, which includes high-grade Ag zones at shallow to intermediate depth (100–1,000 m) that systematically grade into more base metal-rich sulfide veins with increasing depth (Albinson et al, 2001; Simmons et al, 2005; Camprubí and Albinson, 2007; Oliver et al, 2019)
The Variscan orogen resulted from the collision of Gondwana and Laurussia between 400 and 340 Ma (Kroner et al, 2010)
Summary
Intermediate sulfidation Ag-Pb-Zn epithermal systems are a major source of Ag and contain economic amounts of Au, Zn, Pb, and Cu (Simmons et al, 2005). Many of the wellknown examples of this particular ore deposit type are located in the Sierra Madre Occidental of Mexico, e.g., Fresnillo, Tayoltita, and Pachuca-Real del Monte (Simmons, 1991; Albinson et al, 2001; Camprubí and Albinson, 2007), with similar epithermal deposits occurring in Peru (Petersen et al, 1977; Candiotti de los Rios et al, 1990; Baumgartner et al, 2008; Rottier et al, 2018) and Bolivia (Phillipson and Romberger, 2004; Arce Burgoa, 2009), as well as in Spain (Concha et al, 1992), Australia (Oliver et al, 2019), and elsewhere (Sillitoe and Hedenquist, 2003) Many of these deposits have a distinct vertical and lateral zoning, which includes high-grade Ag zones at shallow to intermediate depth (100–1,000 m) that systematically grade into more base metal-rich sulfide veins with increasing depth (Albinson et al, 2001; Simmons et al, 2005; Camprubí and Albinson, 2007; Oliver et al, 2019). In the Cenozoic, the formation of the Eger Graben rift resulted in the exhumation of the Variscan basement and associated hydrothermal deposits (Ziegler, 1990; Ziegler and Dèzes, 2007)
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